University of Portland Pilot Scholars Biology Faculty Publications and Presentations Biology 2008 The aC use of Autotomy in a Stick Insect: Predation Versus Molting Tara Lynne Maginnis University of Portland, [email protected] Follow this and additional works at: http://pilotscholars.up.edu/bio_facpubs Part of the Entomology Commons Citation: Pilot Scholars Version (Modified MLA Style) Maginnis, Tara Lynne, "The aC use of Autotomy in a Stick Insect: Predation Versus Molting" (2008). Biology Faculty Publications and Presentations. 3. http://pilotscholars.up.edu/bio_facpubs/3 This Journal Article is brought to you for free and open access by the Biology at Pilot Scholars. It has been accepted for inclusion in Biology Faculty Publications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact [email protected]. 126 Florida Entomologist 91(1) March 2008 AUTOTOMY IN A STICK INSECT (INSECTA: PHASMIDA): PREDATION VERSUS MOLTING TARA LYNNE MAGINNIS The University of Montana, Division of Biological Sciences, Missoula, MT 59812 Current address: St. Edward’s University, Department of Biology, 3001 South Congress Ave., Austin, TX 78704 Autotomy, or appendage loss, is common in later), at which time they were removed and in- many animals, including reptiles, amphibians, spected for leg loss and/or evidence of regenera- mammals, birds, fish, echinoderms, crustaceans, tion. Nymphs always commence regeneration af- spiders, and insects (see Maginnis 2006a; Flem- ter autotomy, and regenerated legs are always ing et al. 2007 for reviews). In arthropods, there smaller than non-regenerated legs (Bordage 1905; are 2 hypotheses for this phenomenon. First, Ramme 1931; Carlberg 1992; Maginnis 2006b). limbs may be lost through predation attempts; if The results revealed that 17.3% of adults a predator were to grab a leg instead of the body, within the predator-excluded trees were missing the animal can shed the leg and flee to escape pre- and/or regenerated at least one leg during develop- dation (McVean 1982; Carlberg 1986; Formanow- ment (n = 112; 4 of the 10 trees were destroyed by icz 1990; Robinson et al. 1991). And second, limbs cattle (100 individuals), 29 individuals died, 6 in- can be shed during complications with molting. dividuals were still nymphs, and 3 individuals As a result of having a skeleton on the outside of were unaccounted for (original n = 250 - 100 - 29 - the body, arthropods must repeatedly shed their 6 - 3 = 112). This suggests that both predation at- old exoskeleton and replace it with a new one. tempts and complications with molting play im- During this complicated process, jointed append- portant roles as causes of autotomy in this popula- ages can become stuck in the old cuticle and must tion. Interestingly, the rate of autotomy and/or re- be shed to survive (Bedford 1978; Foelix 1982; generation in the enclosed trees, presumably free Carlberg 1986; Robinson et al. 1991; Brock 1999). from predation, was nearly half of the observed Although many taxa within the phylum Ar- rate in a sampled ‘natural’ population (~40%, same thropoda experience autotomy, Phasmida is the species and study location, Maginnis & Maginnis only order within the class Insecta that regularly 2007). This might suggest that ~20% of individuals sheds and regenerates lost legs (Borrer et al. in a phasmid population experience complications 1992). As such, it is important to identify the selec- with molting at some point during their lifetime, tive pressures for autotomy in this group. In the and predation attempts double that base rate of laboratory, legs are lost to molting complications autotomy. That is, perhaps half of the cases of au- approximately 30% of the time (Maginnis 2006b). totomy might be due to molting complications, and In the field, a sampled population showed approx- half might be due to predation. It is also worth not- imately 40% of individuals with missing or regen- ing that atypical predation attempts could have erated legs (Maginnis & Maginnis 2007). How- taken place in this experiment; the “anti-preda- ever, these rates offer no insight into the selective tion” design in this study was aimed at organismal pressures behind autotomy. The goal of this study predators such as birds and mantids, yet legs was to determine the effects of predation attempts could have been lost to the viscous surface of fresh and/or molting complications on rates of leg loss in sap flows within the enclosed trees. a population of Didymuria violescens, the spur- While there is little doubt that all arthropods legged phasmatid, native to south eastern Austra- capable of autotomy experience high selective lia (Campbell & Hadlington 1967). pressure due to predation, 2 factors make the Because it is impossible to control molting com- cause of autotomy in stick insects particularly in- plications, ‘predator-free’ environments were cre- teresting. First, their overall shape; their long and ated in Bago State Forest, New South Wales, Aus- thin legs that are so effective at conferring crypsis tralia. Ten eucalyptus trees (Eucalyptus radiata, could lead to higher rates of molting complications their food source)—all approximately 7 m high than arthropods with shorter, stouter legs. Sec- and 68 centimeters in diameter breast height ond, their exceptional crypsis may lead to higher (DBH)—were covered in mosquito netting. Before rates of ‘misses’; a well camouflaged prey item, a sealing each tree, all visible animals were manu- predator might easily mistake a leg for a body, and ally removed and 25 first instars were subse- thus lead to more missed predation attempts rel- quently placed in each tree (for a total of 250 indi- ative to other less well camouflaged animals. viduals). The sexes of this species were indistin- In conclusion, recognition of the selective pres- guishable at this time, so no effort was made to sures of autotomy on phasmids may provide in- perform the experiment with an equal number of sight as to why they are the only order of insects males and females. Individuals were kept con- that regularly shed and regenerate lost legs. This, tained until maturity (approximately 3 months in combination with future work on the develop- Scientific Notes 127 mental mechanisms and tradeoffs associated plague numbers in forests of south eastern Austra- with autotomy and regeneration, promise to bring lia. Forestry Commission Note No. 20. Forestry a better understanding of this phenomena. Commission, New South Wales. I thank Lawrence Maginnis and the late Alex CARLBERG, U. 1986. Thanatosis and autotomy as de- Marshall for help collecting these data, and Dr. fense in Baculum sp. 1 (Insecta: Phasmida). Zool. Anz. 217: 39-53. Allan Hook for assistance with an earlier version CARLBERG, U. 1992. Cost of autotomy in the Phasmida of this manuscript. This research was funded by species with low autotomy frequency. Zool. Anz. 228: the National Science Foundation (DIG# 0309038). 229-237. FOELIX, R. F. 1982. Biology of Spiders. Harvard Univer- SUMMARY sity Press, Cambridge, MA. FLEMING, P. A, D. SACCAGGI, AND P. W. BATEMAN. 2007. Autotomy is common in many arthropods, and Leave it all behind: an evolutionary and taxonomic can be due to predation avoidance and/or compli- perspective of autotomy in invertebrates. Biol. Rev. cations during molting events. This study evalu- 82: 481-510. ated the effects of both on rates of leg autotomy in JUANES, F., AND L. D. SMITH. 1995. The ecological con- a population of Didymuria violescens, the spur- sequences of limb damage and loss in decapod crus- taceans: a review and prospectus. J. Exp. Marine legged phasmatid. The results suggest that in Biol. and Ecol. 193: 197-223. natural habitats, these insects experience leg loss MAGINNIS, T. L. 2006a. The costs of autotomy and re- due to molting complications approximately 20% generation in animals: a review and framework for of the time. future research. Behavioral Ecol. 17: 857-872. MAGINNIS, T. L. 2006b. 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